A format string consists of zero or more “format units.” A format unit
describes one Python object; it is usually a single character or a parenthesized
sequence of format units. With a few exceptions, a format unit that is not a
parenthesized sequence normally corresponds to a single address argument to
these functions. In the following description, the quoted form is the format
unit; the entry in (round) parentheses is the Python object type that matches
the format unit; and the entry in [square] brackets is the type of the C
variable(s) whose address should be passed.

These formats allow accessing an object as a contiguous chunk of memory.
You don’t have to provide raw storage for the returned unicode or bytes
area.

In general, when a format sets a pointer to a buffer, the buffer is
managed by the corresponding Python object, and the buffer shares
the lifetime of this object. You won’t have to release any memory yourself.
The only exceptions are es, es#, et and et#.

However, when a Py_buffer structure gets filled, the underlying
buffer is locked so that the caller can subsequently use the buffer even
inside a Py_BEGIN_ALLOW_THREADS block without the risk of mutable data
being resized or destroyed. As a result, you have to callPyBuffer_Release() after you have finished processing the data (or
in any early abort case).

For all # variants of formats (s#, y#, etc.), the type of
the length argument (int or Py_ssize_t) is controlled by
defining the macro PY_SSIZE_T_CLEAN before including
Python.h. If the macro was defined, length is a
Py_ssize_t rather than an int. This behavior will change
in a future Python version to only support Py_ssize_t and
drop int support. It is best to always define PY_SSIZE_T_CLEAN.

Convert a Unicode object to a C pointer to a character string.
A pointer to an existing string is stored in the character pointer
variable whose address you pass. The C string is NUL-terminated.
The Python string must not contain embedded null code points; if it does,
a ValueError exception is raised. Unicode objects are converted
to C strings using 'utf-8' encoding. If this conversion fails, a
UnicodeError is raised.

Note

This format does not accept bytes-like objects. If you want to accept
filesystem paths and convert them to C character strings, it is
preferable to use the O& format with PyUnicode_FSConverter()
as converter.

Changed in version 3.5: Previously, TypeError was raised when embedded null code points
were encountered in the Python string.

This format accepts Unicode objects as well as bytes-like objects.
It fills a Py_buffer structure provided by the caller.
In this case the resulting C string may contain embedded NUL bytes.
Unicode objects are converted to C strings using 'utf-8' encoding.

Like s*, except that it doesn’t accept mutable objects.
The result is stored into two C variables,
the first one a pointer to a C string, the second one its length.
The string may contain embedded null bytes. Unicode objects are converted
to C strings using 'utf-8' encoding.

This format converts a bytes-like object to a C pointer to a character
string; it does not accept Unicode objects. The bytes buffer must not
contain embedded null bytes; if it does, a ValueError
exception is raised.

Changed in version 3.5: Previously, TypeError was raised when embedded null bytes were
encountered in the bytes buffer.

Convert a Python Unicode object to a C pointer to a NUL-terminated buffer of
Unicode characters. You must pass the address of a Py_UNICODE
pointer variable, which will be filled with the pointer to an existing
Unicode buffer. Please note that the width of a Py_UNICODE
character depends on compilation options (it is either 16 or 32 bits).
The Python string must not contain embedded null code points; if it does,
a ValueError exception is raised.

Changed in version 3.5: Previously, TypeError was raised when embedded null code points
were encountered in the Python string.

This format accepts any object which implements the read-write buffer
interface. It fills a Py_buffer structure provided by the caller.
The buffer may contain embedded null bytes. The caller have to call
PyBuffer_Release() when it is done with the buffer.

This variant on s is used for encoding Unicode into a character buffer.
It only works for encoded data without embedded NUL bytes.

This format requires two arguments. The first is only used as input, and
must be a constchar* which points to the name of an encoding as a
NUL-terminated string, or NULL, in which case 'utf-8' encoding is used.
An exception is raised if the named encoding is not known to Python. The
second argument must be a char**; the value of the pointer it
references will be set to a buffer with the contents of the argument text.
The text will be encoded in the encoding specified by the first argument.

PyArg_ParseTuple() will allocate a buffer of the needed size, copy the
encoded data into this buffer and adjust *buffer to reference the newly
allocated storage. The caller is responsible for calling PyMem_Free() to
free the allocated buffer after use.

This variant on s# is used for encoding Unicode into a character buffer.
Unlike the es format, this variant allows input data which contains NUL
characters.

It requires three arguments. The first is only used as input, and must be a
constchar* which points to the name of an encoding as a
NUL-terminated string, or NULL, in which case 'utf-8' encoding is used.
An exception is raised if the named encoding is not known to Python. The
second argument must be a char**; the value of the pointer it
references will be set to a buffer with the contents of the argument text.
The text will be encoded in the encoding specified by the first argument.
The third argument must be a pointer to an integer; the referenced integer
will be set to the number of bytes in the output buffer.

There are two modes of operation:

If *buffer points a NULL pointer, the function will allocate a buffer of
the needed size, copy the encoded data into this buffer and set *buffer to
reference the newly allocated storage. The caller is responsible for calling
PyMem_Free() to free the allocated buffer after usage.

If *buffer points to a non-NULL pointer (an already allocated buffer),
PyArg_ParseTuple() will use this location as the buffer and interpret the
initial value of *buffer_length as the buffer size. It will then copy the
encoded data into the buffer and NUL-terminate it. If the buffer is not large
enough, a ValueError will be set.

In both cases, *buffer_length is set to the length of the encoded data
without the trailing NUL byte.

Store a Python object (without any conversion) in a C object pointer. The C
program thus receives the actual object that was passed. The object’s reference
count is not increased. The pointer stored is not NULL.

O! (object) [typeobject, PyObject *]

Store a Python object in a C object pointer. This is similar to O, but
takes two C arguments: the first is the address of a Python type object, the
second is the address of the C variable (of type PyObject*) into which
the object pointer is stored. If the Python object does not have the required
type, TypeError is raised.

O& (object) [converter, anything]

Convert a Python object to a C variable through a converter function. This
takes two arguments: the first is a function, the second is the address of a C
variable (of arbitrary type), converted to void*. The converter
function in turn is called as follows:

status=converter(object,address);

where object is the Python object to be converted and address is the
void* argument that was passed to the PyArg_Parse*() function.
The returned status should be 1 for a successful conversion and 0 if
the conversion has failed. When the conversion fails, the converter function
should raise an exception and leave the content of address unmodified.

If the converter returns Py_CLEANUP_SUPPORTED, it may get called a
second time if the argument parsing eventually fails, giving the converter a
chance to release any memory that it had already allocated. In this second
call, the object parameter will be NULL; address will have the same value
as in the original call.

Tests the value passed in for truth (a boolean predicate) and converts
the result to its equivalent C true/false integer value.
Sets the int to 1 if the expression was true and 0 if it was false.
This accepts any valid Python value. See Truth Value Testing for more
information about how Python tests values for truth.

The object must be a Python sequence whose length is the number of format units
in items. The C arguments must correspond to the individual format units in
items. Format units for sequences may be nested.

It is possible to pass “long” integers (integers whose value exceeds the
platform’s LONG_MAX) however no proper range checking is done — the
most significant bits are silently truncated when the receiving field is too
small to receive the value (actually, the semantics are inherited from downcasts
in C — your mileage may vary).

A few other characters have a meaning in a format string. These may not occur
inside nested parentheses. They are:

|

Indicates that the remaining arguments in the Python argument list are optional.
The C variables corresponding to optional arguments should be initialized to
their default value — when an optional argument is not specified,
PyArg_ParseTuple() does not touch the contents of the corresponding C
variable(s).

$

PyArg_ParseTupleAndKeywords() only:
Indicates that the remaining arguments in the Python argument list are
keyword-only. Currently, all keyword-only arguments must also be optional
arguments, so | must always be specified before $ in the format
string.

New in version 3.3.

:

The list of format units ends here; the string after the colon is used as the
function name in error messages (the “associated value” of the exception that
PyArg_ParseTuple() raises).

;

The list of format units ends here; the string after the semicolon is used as
the error message instead of the default error message. : and ;
mutually exclude each other.

Note that any Python object references which are provided to the caller are
borrowed references; do not decrement their reference count!

Additional arguments passed to these functions must be addresses of variables
whose type is determined by the format string; these are used to store values
from the input tuple. There are a few cases, as described in the list of format
units above, where these parameters are used as input values; they should match
what is specified for the corresponding format unit in that case.

For the conversion to succeed, the arg object must match the format
and the format must be exhausted. On success, the
PyArg_Parse*() functions return true, otherwise they return
false and raise an appropriate exception. When the
PyArg_Parse*() functions fail due to conversion failure in one
of the format units, the variables at the addresses corresponding to that
and the following format units are left untouched.

Parse the parameters of a function that takes both positional and keyword
parameters into local variables. The keywords argument is a
NULL-terminated array of keyword parameter names. Empty names denote
positional-only parameters.
Returns true on success; on failure, it returns false and raises the
appropriate exception.

Function used to deconstruct the argument lists of “old-style” functions —
these are functions which use the METH_OLDARGS parameter parsing
method, which has been removed in Python 3. This is not recommended for use
in parameter parsing in new code, and most code in the standard interpreter
has been modified to no longer use this for that purpose. It does remain a
convenient way to decompose other tuples, however, and may continue to be
used for that purpose.

A simpler form of parameter retrieval which does not use a format string to
specify the types of the arguments. Functions which use this method to retrieve
their parameters should be declared as METH_VARARGS in function or
method tables. The tuple containing the actual parameters should be passed as
args; it must actually be a tuple. The length of the tuple must be at least
min and no more than max; min and max may be equal. Additional
arguments must be passed to the function, each of which should be a pointer to a
PyObject* variable; these will be filled in with the values from
args; they will contain borrowed references. The variables which correspond
to optional parameters not given by args will not be filled in; these should
be initialized by the caller. This function returns true on success and false if
args is not a tuple or contains the wrong number of elements; an exception
will be set if there was a failure.

This is an example of the use of this function, taken from the sources for the
_weakref helper module for weak references:

Create a new value based on a format string similar to those accepted by the
PyArg_Parse*() family of functions and a sequence of values. Returns
the value or NULL in the case of an error; an exception will be raised if
NULL is returned.

Py_BuildValue() does not always build a tuple. It builds a tuple only if
its format string contains two or more format units. If the format string is
empty, it returns None; if it contains exactly one format unit, it returns
whatever object is described by that format unit. To force it to return a tuple
of size 0 or one, parenthesize the format string.

When memory buffers are passed as parameters to supply data to build objects, as
for the s and s# formats, the required data is copied. Buffers provided
by the caller are never referenced by the objects created by
Py_BuildValue(). In other words, if your code invokes malloc()
and passes the allocated memory to Py_BuildValue(), your code is
responsible for calling free() for that memory once
Py_BuildValue() returns.

In the following description, the quoted form is the format unit; the entry in
(round) parentheses is the Python object type that the format unit will return;
and the entry in [square] brackets is the type of the C value(s) to be passed.

The characters space, tab, colon and comma are ignored in format strings (but
not within format units such as s#). This can be used to make long format
strings a tad more readable.

Pass a Python object untouched (except for its reference count, which is
incremented by one). If the object passed in is a NULL pointer, it is assumed
that this was caused because the call producing the argument found an error and
set an exception. Therefore, Py_BuildValue() will return NULL but won’t
raise an exception. If no exception has been raised yet, SystemError is
set.

S (object) [PyObject *]

Same as O.

N (object) [PyObject *]

Same as O, except it doesn’t increment the reference count on the object.
Useful when the object is created by a call to an object constructor in the
argument list.

O& (object) [converter, anything]

Convert anything to a Python object through a converter function. The
function is called with anything (which should be compatible with void*) as its argument and should return a “new” Python object, or NULL if an
error occurred.